Generally, self-propelled vibratory compacting machines are roughly classified into roller type machines and crawler type machines (with tracks or crawlers). Most of the roller type machines are designed for dedicated applications, and include vibrators for generating vertical vibrations (hereinafter called "vertical vibrators"). With the roller type vibratory compacting machine, its roller has a substantially line contact with the surface of the ground, and applies a large surface pressure per unit area of the ground. Therefore, such a machine suffers from the following problems. First of all, when compacting soft ground, the large surface pressure per unit area tends to distort the surface layer of the soft ground, which means that the surface layer cannot be compacted. Secondly, when compacting hard ground, a surface layer of such hard ground can be compacted, but there can be a large disparity between the rigidity of the surface layer and that of a deep layer, so that the deep layer cannot be completely compacted. Basically, the deep layer can be compacted by increasing the weight and the vertical vibrations of the compacting machine. However, if such measures are taken and great pressure per unit area, larger than the pressure per unit area that the particular type of ground can endure, is applied, the surface layer of the hard ground is not compacted, as is the case with soft ground. In other words, the hard ground can be compacted to a depth of approximately 30 cm at most.
Japanese Patent Publication No. Hei 5-41761 discloses a "Roller type vibratory compacting machine having a large eccentric weight and a small eccentric weight that rotate at low and high speeds, respectively", and pays particular attention to the fact that "the machine can compact a deep layer of the ground by rotating the large eccentric weight of a vertical vibrator at a low speed and with large amplitude, while the surface layer can be compacted by rotating the small weight at a high speed and with small amplitude".
Japanese Utility Model Laid-open No. Hei 1-119407 discloses a "Roller type vibratory compacting machine", and pays particular attention to the fact that "when a roller is horizontally vibrated, moisture and air is discharged from the ground, which enhances compacting of the ground," and that "horizontal vibrations are less hazardous to the environment than vertical vibrations." In the Utility Model, a horizontal vibrator is employed in place of a vertical vibrator.
Contrary to the roller type vibratory compacting machine, a crawler type vibratory compacting machine comes into substantial surface contact with the ground. Although the surface pressure per unit area is small, such a machine can apply the pressure to a wide area of the ground surface. In other words, the disparity between the pressure applied to a unit area of the surface layer of the ground and the pressure applied to a unit area of the deep layer is much smaller than that of the roller type vibratory compacting machine. Further, the surface pressure is applied to the ground for a longer period of time during forward and backward movements of the crawler type vibratory compacting machine, compared with the roller type vibratory compacting machine, i.e. the surface pressure lasts far longer than in the roller type machine. Therefore, the crawler type machine can carry out a uniform compacting of the ground from the surface layer to the deep layer. The crawler type machine is free from the first and second problems, and is advantageous in that it can compact a deep layer of the ground (approximately 1 meter deep). The following proposals have been made in publications related to crawler type vibratory compacting machines.
(1) Japanese Patent Laid-open No. Sho 58-135231 discloses a "Crawler type shovel having vertical vibrators attached to left and right track frames."
(2) Japanese Patent Laid-open No. Sho 61-257506 discloses a crawler type vibratory compacting machine which comprises: (A) an upper structure including a power source; (B) a lateral plate positioned under the upper structure in order to support it via a spring; (C) left and right side plates extending from left and right ends of the lateral plate; (D) a bottom plate arranged between lower sides of the left and right side plates, and supporting a vertical vibrator on the upper surface thereof; (E) drive sprockets and driving wheels positioned at the front and rear parts of the bottom plate, and rotatably supported by front and rear ends of the left and right side plates, respectively; and (F) left and right tracks wound around the drive sprockets and driving wheels and an outer surface of the bottom plate.
(3) Japanese Patent Publication No. Hei 7-23609 discloses a "Crawler type vibratory compacting machine", in which a vertical vibrator is installed on the frame of a self-propelled crawler type vehicle, and energy for running, steering and vibrating actions is supplied via flexible energy supply tubes from a power unit provided at a remote location.
However, the foregoing self-propelled vibratory compacting machines still suffer from the following problems.
Specifically, there are the foregoing first problem that roller type machines cannot compact soft ground, and the second problem that the roller type machines can only compact hard ground to a depth of approximately 30 cm at maximum. Japanese Patent Publication No. Hei 5-41761 discloses that "the self-propelling vibratory compacting machine can compact both the surface layer and the deep layer of the ground", but does not specify a depth of the ground that can be compacted, and leaves the first problem unsolved.
Japanese Utility Model Laid-open No. Hei 1-119407 leaves the first problem unsolved. The horizontal vibrations of the horizontal vibrator allow moisture and air to be discharged from "the ground where the roller is in line contact with the ground which is free from the roller". However, the ground which is free from the roller includes the surface layer of the ground that has been already compacted by the roller. Moisture and air will also be forced into the already compacted surface layer. Therefore, the already compacted surface layer increases its moisture content, and may be distorted by the vertical vibrations transferred from the ground being in line contact with the roller, and thus softened.
The foregoing crawler type vibratory compacting machines have the following problems. Japanese Patent Laid-open No. Sho 58-135231 relates to a power shovel including a pair of crawler type tracks attached to the left and right sides of the vehicle body. One forward or backward movement of the shovel cannot compact the ground which is not in contact with the left track or the right track. Further, since no springs are used, the vertical vibrations of the vibrator are transmitted to the operator and the vehicle body, which may be uncomfortable to the operator, adversely affect the operator's health, and shorten the life of various components of the machine.
The problem related to the absence of springs does not affect Japanese Patent Laid-open No. Sho 61-257506 in which the upper structure having the power unit is installed on springs. Further, the bottom plate receives vertical vibrations and the weight of the vehicle body, so that large vertical vibrations can be produced. However, the vehicle is moved back and forth by causing the track to slide on the rear surface of the bottom plate in response to the rotation of a drive sprocket. Therefore, the drive sprocket has to produce a driving force that can overcome the sliding resistance of the track. Further, in order to compact the ground, downward vibrations act in order to periodically press the bottom plate against the track, and to periodically increase the sliding friction between the rear surface of the bottom plate and the upper surface of the track. This periodical large sliding friction serves as a damping force for the drive sprocket, which shortens the lifespan of the drive sprocket and the power transmission system therefor. In addition, there is a problem in that both the bottom plate and the track will be excessively worn due to the sliding.
Japanese Patent Publication No. Hei 7-23609 is free from the problem of the power unit being damaged by vibrations, since the power unit is positioned far from the machine body. However, a pair of the crawler type tracks are spaced apart, so that one forward or backward movement of the machine cannot compact the ground that is not in contact with either of the tracks.
The existing machines have been respectively reviewed. In summary, the existing self-propelled vibratory compacting machines have not made sufficient use of the merits of roller type vibratory compacting machines and the crawler type machines, and have not enhanced the advantages of the crawler type machines.